The present invention relates in general to filtration devices and, more particularly, to hollow fiber membranes and a method of construction thereof.
Membranes are linings or partitions which are able to divide or separate two phases, e.g., a liquid phase from a gas phase, a solid from a liquid, or multiple components of a liquid from each other. These membranes are typically formed from polymers and are semipermeable, allowing a transfer between the phases which are to be separated. The specific physical shape or form of the membranes can vary, and can include flat sheets, tubular membranes, and hollow fibers. The specific use to which the membrane is to be put dictates the form selected for its construction. Membranes in the form of hollow fibers are currently used in a variety of applications, including dialysis, gas separation, ultrafiltration, microfiltration, and nanofiltration.
In forming hollow fiber membranes, the polymer chosen for the membrane is processed, initially, into one of two different types of fiber forming materials. The polymer is either dissolved in a suitable solvent to form a polymer solution, or heated to form a polymer melt. The properties of the particular polymer used, as well as the properties of the desired end product, allow one skilled in the art to determine whether the fiber forming material selected for a particular application will be a polymer solution or a polymer melt. When polymer solutions are utilized, the phase inversion from a liquid phase to a solid phase can be achieved in one of several different ways, including evaporation of the solvent, precipitation of the polymer, cooling of the solution to solidify the polymer, or use of a non-solvent to remove the solvent from the solution. On the other hand, when polymer melts are utilized, the phase inversion is generally thermally induced.
Once the selected polymer is processed into the desired fiber forming material for constructing a hollow fiber membrane, i.e., a polymer solution or a polymer melt, the material is extruded through the annular opening of a spinneret, generally the tube-in-orifice type, to form a hollow core extrudate. Internal bore fluids are often co-extruded within the hollow fiber membrane to form the bore or lumen of the hollow fiber. When the fiber forming material to be processed is a polymer melt, the extruded fiber is solidified by means of a thermally generated phase inversion. This phase inversion is induced by the relatively cool temperature of a coagulation bath into which the hollow fiber is passed subsequent to extrusion, as compared to the temperature of the polymer melt at the time of extrusion, or by the relatively cool temperature of the bore fluid or of the air contacted by the extruded fiber prior to its submersion in a coagulation bath. When the fiber forming material to be processed is a polymer solution, phase inversion can be achieved by evaporating the solvent, leaving the polymer membrane in its final form. If the solvent is not sufficiently volatile, it may be necessary to pass the extruded fiber through a bath containing a non-solvent, which will remove the solvent from the polymer solution, precipitating the polymer in its final form as a hollow fiber membrane.
One of the polymers useful in the production of these hollow fiber membranes is polyvinylidene fluoride, commonly referred to as PVDF. The use of PVDF and other polymers in the production of hollow fibers is described in great detail in U.S. Pat. No. 4,958,733. According to the teachings of this patent, PVDF is processed into a fiber forming material by heating it to about 185.degree. C. to form a polymer melt, and then extruding the melt through a spinneret. An internal bore fluid is co-extruded in the lumen of the fiber, helping to solidify and form the inner core of the hollow fiber membrane. This extruded hollow fiber is then passed through a water bath, having a temperature of about 20.degree. C., where it is allowed to cool and further solidify the membrane. Formation of a hollow fiber membrane according to this patent is thus triggered by a thermally induced phase inversion.
Prior to the present invention, attempts to construct PVDF hollow fiber membranes from polymer solutions were unsuccessful, as the membranes thus formed had insufficient strength to retain the hollow fiber form or to withstand the rigors of the filtration process. In order to construct hollow fiber membranes from PVDF which were strong enough to function adequately, the conventional practice was to increase the concentration of the PVDF in the solution. However, to achieve adequate strength characteristics, the PVDF concentration had to be increased to a point at which it was necessary to heat the polymer in order to put it into solution, effectively turning the solution into a polymer melt. However, the processing of PVDF into a polymer melt and the subsequent thermally induced phase inversion associated with polymer melts tended to compromise certain characteristics of the end product membrane formed.
Thus, while PVDF hollow fiber membranes formed from conventional polymer solutions might have sufficient strength for use as membrane sheets or other types of membranes which have backing elements associated therewith, they lacked the necessary strength to function adequately as hollow fiber membranes which generally do not incorporate backings for imparting additional strength thereto. The fiber itself must possess sufficient rigidity and strength to withstand the rigors of ultrafiltration and other end uses. Therefore, PVDF hollow fiber membranes formed from polymer solutions have not heretofore been commercially desirable.
While PVDF hollow fiber membranes constructed from polymer melts might have sufficient strength to stand alone as hollow fiber membranes, the resultant porosity achieved was not commercially acceptable. The reason for this decreased porosity is thought to be that as the polymer is melted, all gases and voids within it are expelled, maximizing the polymer density. When the melt undergoes phase inversion subsequent to spinning, the resultant fiber membrane has a density at least equal to or greater than that of the starting polymer material. This increased density translates to decreased porosity, a drawback in the membrane industry. Additionally, PVDF membranes formed from polymer melts have primarily symmetrical pore structures, another potentially undesirable feature of hollow fiber membranes.
In addition to these deficiencies found in the prior art, it was previously thought that the size of the pores in the membranes was primarily dependent upon the amount of pore forming agent included in the polymer solution or polymer melt. However, increasing the amount of pore former to provide the desired porosity often resulted in membranes having pores which were too large, with the resultant membrane being too weak to provide effective filtration.